At comparable conceptional ages, preterm infants exhibit different behavioral organization than term infants, show different patterns of interaction with the primary caregiver, and perform differently on some attentional and information processing tasks. Follow-up studies of preterm infants yield a concordance of findings implicating the socioeconomic status and maternal responsivity as significant determinants of developmental status. There is little empirical evidence, however, to describe the mechanisms that account for these findings or to define the relationship between maternal behavior and infant information processing characteristics. The research proposed here will provide a test of hypotheses about the role of functional maternal stimulation in interaction with individual differences in information processing characteristics of preterm and term infants as joint determinants of developmental outcome. Preterm and term infants from middle and lower SES backgrounds who have histories of medical stress and non-stress will be evaluated frequently over their first 18 months of life, using the Neonatal Behavioral Assessment Scale in the first month and the Bayley Schales at 6, 10, 14, and 18 months. Mother-infant interaction observations will be carried out when the infants are newborns and at 3, 6, 10, 14, and 18 months. Laboratory assessments will be made of visual and auditory habituation at 3 and 6 months, auditory-visual integration at 6 and 10 months, relational information processing at 6 and 10 months, and category formation abilities at 10 and 14 months. Social referencing behavior will be asessed at 6 and 10 months and mastery/motivation at 10 and 14 months. Language development will also be assessed at 18 months. In addition, an illness record will be gathered at each home visit, and medical and background information will be collected. Approximately 170 of the total sample of 196 infants will be followed up at 2 1/2 years, using the Stanford-Binet and collecting a language sample for evaluation. It is predicted that the stronger and more efficient the infant's information processing abilities, the less critical the relative responsivity of the environment will be. For infants whose initial condition is stressed and whose information processing skills are weak, however, the quality of the social environment is expected to be a vital factor in developmental outcome.